The present invention relates to a deflective focusing system for a charged particle beam, the system having a magnetic lens and an electrostatic deflector.
In general, deflective focusing systems for charged particle beams (referred to merely as "beams", hereinafter) are widely used in cathode ray tubes, television camera tubes, electron beam processing equipment, electron beam exposure equipment, scanning type electron microscopes, or the like.
For example, as VLSI (Very Large Scale Integrated Circuit) techniques evolve, the development of an electron beam exposure equipment with high speed and high accuracy is strongly desired. In order to realize such an exposure equipment, it is essentially necessary to develop a high-performance deflective focusing system. In an electron beam exposure equipment, a beam produced from an electron gun is shaped to a beam with a square section. This square beam is then demagnified. The demagnified beam is then focused and deflected to project at a desired postion on a target plane or specimen wafer on a table. In the deflective focusing system, aberrations due to the deflection of the beam, i.e., chromatic aberration, astigmatism coma, field curvature and distortion, are required to be small and the landing angle at which a beam is incident to the target must also be small. If the aberrations and the landing angle are large, resolution and accuracy of patterning are lowered. Further, from a viewpoint of the high speed deflection of beam, electrostatic deflection is preferable to magnetic deflection.
Generally speaking, when a beam is focused and deflected by a magnetic focusing field and an electrostatic deflection field which overlap each other and these fields are distribute uniformly over the whole of the deflective focusing space, the aberrations are extremely low and the landing angle is small enough that the beam is incident vertically to an image plane or target.
In the electron beam exposure equipment, however, a demagnifying lens is disposed on the object plane side of the deflective focusing system and a wafer or stage is disposed on the image plane side, so that it is difficult to obtain a completely uniform electromagnetic field over the whole deflective focusing space. There are fringes on the object plane side and the image plane side of the deflective focusing system where the electric field and magnetic field abruptly change. If the electromagnetic field has fringes in this way, electron optical properties of the deflective focusing system are different from the properties in case of the uniform distribution. It follows that both of the aberrations and landing angle increase.
For instance, an in-lens type magnetic deflector is disclosed by J. L. Mauer et al. in "Electron Optics of an Electron-Beam Lithographic System", IBM J. RES. DEVELOP., pp. 514-521, November 1977. This deflector has large aberrations and landing angle and, since magnetic deflection is employed in this deflector, the deflection speed is slow.
There have also been some proposals where a plurality of stages of deflectors are provided and adjusted in a manner so that the deflective aberrations due to the respective deflectors cancel each other out to realize small aberrations and small landing angle throughout the system as a whole. See, for example, "Advanced deflection concept for large area, high resolution e-beam lithography" by H. C. Pfeiffer et al., J. Vac. Sci. Technol., 19(4), November/December 1981, pp. 1058-1063. In the disclosed variable axis lens, four-stage deflectors and one dynamic stigmator are used to reduce the deflective aberrations and landing angle. The deflective aberrations are completely removed and the vertical landing condition is also satisfied by such a multi-stage deflection system. These facts are theoretically proved by T. Hosokawa in "Systematic elimination of third order aberrations in electron beam scanning system", Optik, Vol. 56, No. 1 (1980), pp. 31-30.
In this case, however, the number of power sources for driving deflectors is increased because of the multi-stage deflectors. Since a power source of such a deflective focusing system is very expensive, the cost of this multi-stage deflection system is very complicated. In addition, high manufacturing techniques are required, as the number of deflection stages is increased. This requirement also constitutes a barrier against the realization of a multi-stage deflection system.